1. Technical Field of the Invention
The present invention relates to reflectometer technology and, in particular, to a method and apparatus for detecting and measuring color shades with a relatively high degree of accuracy. Where the color shades are indicative of a certain measurable quantity or quality, the present invention further relates to method and apparatus for converting the detected color shade into a corresponding quantity or quality measurement.
2. Description of Related Art
Presently, the only approved method for home monitoring of blood chemistry requires drawing blood by using a lance, usually by sticking a finger, and placing a drop of blood on a chemical strip. The resulting chemical reaction causes a change in the color of the strip with that change being read by a desk-top reflectance meter to provide an indication of blood sugar level. Another method also requires drawing blood, placing a drop of blood on a disposable printed circuit (PC) board, and measuring the electrical response of the blood to detect blood sugar level. Some attempts to use infrared techniques to look through the skin to make blood sugar determinations have proven to be less reliable and too expensive for commercial application.
Diabetics who need to control their insulin level via diet or insulin injection may test themselves five or six times per day, the frequency recommended by the American Diabetes Association. Some may choose to test less often than recommended to avoid the unpleasantness associated with drawing blood. There is accordingly a considerable amount of interest in the development of procedures for making blood sugar level determinations that avoid any need for inflicting injury to the patient.
One technology which has demonstrated accurate and repeatable results employs a transdermal patch to detect and measure blood sugar levels. This transdermal patch technology utilizes a transport mechanism to extract glucose related analytes from the skin (such as those found in interstitial fluid) for transport to a sensitive membrane. At this membrane, a chemical or biological reaction with the extracted analyte occurs to develop a color indicator thereon whose color and shade can be related to glucose levels. One such patch apparatus is disclosed by Aronowitz, et al., in commonly assigned, U.S. patent application Ser. No. 08/929,262, filed Sep. 11, 1997, the disclosure of which is incorporated herein by reference. Another patch-type glucose measurement technology is taught by Peck in U.S. Pat. No. 4,821,733, the disclosure of which is incorporated herein by reference.
With respect to at least that transdermal detection mechanism disclosed by Aronowitz, et al., the extracted analytes which are indicative of widely varying blood sugar levels unfortunately produce only very slight changes in developed color shade. In many instances, the difference between developed color shade for an acceptable and an unacceptable blood sugar level cannot be accurately and repeatably detected by the naked eye. To obtain the non-invasive benefits of transdermal glucose measurement technology while ensuring measurement accuracy in what may comprise a life critical testing procedure, it is therefore imperative that the fallible human activity of color shade evaluation and comparison be eliminated from the testing and measurement process.
There is accordingly a need for an ultra-sensitive meter capable of accurately resolving the full range of developed subtle color shade changes produced as a result of transdermal patch extraction and processing of certain analytes of interest. Preferably, the meter should be small, lightweight and portable (hand held). Beyond the obvious requirements for improved sensitivity to subtle differences in color shade, this meter should account for the effects of portability which are adverse to reading accuracy such as background light changes, temperature changes, and unsteady hand-held operation (for example, due to device pressure variation, rotation, and movement), and which are not normally associated with the desk-top meters that are widely employed for measuring blood sugar levels on test strips.
The present invention comprises reflectometer for detecting and measuring subtle changes in color and shade of color. In general, a pulsating light source illuminates a target surface which possesses a certain color and shade of color. An optical detection circuit synchronously detects light that is reflected from the target surface and generates an output signal whose voltage is indicative of the color and shade of the target surface. This voltage is then processed to make an evaluation and identification of any measurable quantity or quality that is represented by the detected color or shade of color.
More specifically, a modulated light source emits light to illuminate the colored target surface, where the specific color or shade of color is indicative of a certain measurable quantity or quality (such as an analyte concentration). The modulated light that is reflected from the target surface is detected by an optical detector. The output signal from the optical detector is differentially amplified to produce an AC output signal indicative of the color and shade of the target surface. The output signal from the optical detector is further processed and fed back to the optical detector to compensate for any shift in the DC level of the AC output signal caused by the detection of ambient light or the influence of other external factors. The output signal from the differential amplifier is then demodulated by a synchronous detector to produce a substantially steady DC voltage that is indicative of the color or shade of color at the target surface. This DC voltage is converted to a corresponding digital value, and that digital value is converted using a look-up table or other mathematical formula into a corresponding quantity or quality measurement.